This invention relates generally to micro-electromechanical systems (MEMS), and more specifically, to multi-frequency MEMS devices capable of simultaneous fabrication.
Micro-electromechanical systems (MEMS) integrate electrical and mechanical components on the same substrate, for example, a silicon substrate, using microfabrication technologies. The electrical components are fabricated using integrated circuit processes, while the mechanical components are fabricated using micromachining processes that are compatible with the integrated circuit processes. This combination makes it possible to fabricate an entire system on a chip using standard manufacturing processes.
One common application of MEMS devices is in the design and manufacture of sensor devices. The mechanical portion of the sensor device provides the sensing capability, while the electrical portion of the sensor device processes the information received from the mechanical portion. One example of a MEMS device is a gyroscope. Some inertial measurement units (IMUs) incorporate one or more MEMS gyroscopes.
One known type of MEMS gyroscope uses a vibrating element to sense angular rate through the detection of a Coriolis acceleration. The vibrating element is put into oscillatory motion along the X-axis (motor axis), which is parallel to the substrate, in a resonant mode of vibration referred to as a motor mode. Once the vibrating element is put in motion, it is capable of detecting angular rates induced by the substrate being rotated about the Z-axis (input axis), which is perpendicular to the substrate. Coriolis acceleration occurs along the Y-axis (sense axis), which is perpendicular to both the X-axis and the Z-axis, causing oscillatory motion along the Y-axis, in a resonant mode referred to as a sense mode. The amplitude of oscillation of the sense mode is proportional to the angular rate of the substrate. Such a MEMS gyroscope is sometimes referred to as a z-axis gyroscope or out-of-plane gyroscope. As used herein a z-axis gyroscope is a gyroscope which measures rotation about an axis perpendicular to a substrate surface. Another type of MEMS gyroscope, known as an in-plane gyroscope, measures rotation about the Y-axis, by detecting sense mode motion along the Z-axis caused by the Coriolis acceleration.
In a specific IMU, which incorporates three MEMS gyroscopes, the three gyroscopes should have different motor frequencies, and the output voltage per unit angular rotation rate, sometimes referred to as scale factor, should be about the same. If the motor frequencies of all three gyroscopes are not different, then a noise-to-signal ratio of the IMU output signal, as measured by angular random walk of the IMU, is higher. Having the same scale factor for all three devices is most easily obtained if a motor-sense frequency separation is about the same for all three gyroscopes. For ease in fabrication, it is preferred to provide all three gyroscopes from the same wafer without requiring additional fabrication process steps to provide the three different motor frequencies while also retaining a motor-sense frequency separation which is the same for all three gyroscopes.